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Various combinations of RNA segments can result in a new subtype of virus (known as antigenic shift) that might have the capability to preferentially infect humans but still show characteristics unique to the swine influenza virus. It's even feasible to consist of RNA strands from birds, swine, and human influenza viruses into 1 virus if a cell becomes infected with all 3 varieties of influenza (for example, two bird flu, 3 swine flu, and 3 human flu RNA segments to produce a viable eight-segment new kind of flu viral genome). Formation of a new viral type is considered to be antigenic shift; modest changes in an individual RNA segment in flu viruses are termed antigenic drift and result in minor changes in the virus. However, these can accumulate over time to produce sufficient minor changes that cumulatively change the virus antigenic makeup over time (usually years).
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